This invention relates to a semiconductor device prevented from changes of internal stress in a silicon thin film and generation of crystal defects caused by the changes of internal stress, and processes for producing the same, as well as to processes for producing a silicon thin film and a chemical vapor deposition apparatus suitable for forming such a silicon thin film.
In the production of semiconductor devices, a silicon thin film is used as electrodes and/or a wiring material. Since the silicon thin film is a semiconductor material, it is necessary to reduce electric resistance when used as a wiring material. In general, it is doped with an element of group III or V (e.g. B, P, As, etc.) by diffusion. In the doping with such an impurity, there has been employed thermal diffusion from film surface or ion implantation.
Recently, since the structure of semiconductor devices is complicated, a level difference of surfaces on which the thin film is to be deposited is made as small as possible in order to improve evenness of deposition of the thin film. Thus, there is a tendency to reduce the film thickness of various thin films including a silicon thin film. When the film thickness is reduced, there arise problems such as contamination of an underlying film with a dopant, concentration and uneven deposition of a dopant near the interface of underlying film, and the like, when the thermal diffusion from film surface or the ion implantation is employed. In order to solve such problems, an in-situ doping technique wherein an impurity is doped simultaneously at the time of deposition of a silicon thin film is proposed and used for producing products.
As processes for depositing a silicon thin film, there are known a process which comprises depositing silicon in an amorphous state, followed by polycrystallization by heat treatment, and a process for depositing in a polycrystalline state from the beginning. Generally speaking, since there is a tendency to enlarge crystal grain size in the case of deposition in an amorphous state, followed by polycrystallization by heat treatment, it is preferable to form a polycrystalline silicon film by this process in order to attain low electric resistance of the thin film. Therefore, there is widely used a process for forming a polycrystalline silicon film comprising depositing amorphous silicon doped with an impurity simultaneously, followed by polycrystallization by heat treatment. Such a technique is disclosed, for example, in Japanese Patent Unexamined Publication No. (JP-A) 62-54423 and 4-137724.
But, according to such a technique, there are following problems. When an amorphous (including a fine crystalline state) silicon thin film is crystallized by heat treatment, it is generally known that crystal nucleuses are grown from the interface between the silicon thin film and the underlying film. Therefore, the state of crystal growth is often changed (by, for example, generating density and generating temperature of crystal nucleuses, crystal grain size, or growing crystal plane indices) depending on an impurity concentration or its distribution in the amorphous silicon film near the interface of underlying film.
Further, at the time of crystallization reaction, since the volume of thin film is changed, the internal stress state in the film is also changed greatly. Further, the direction of stress (i.e. tensile strength or compression stress) generated at the time of crystallization is greatly changed by growing crystal state. As a result, there arise various problems in that generated internal stress in the silicon thin film becomes greater, or in a wafer on which the thin film is deposited, the internal stress in the thin film and growing crystal planes are differentiated, the degree of concentration of stress generated near end portions of the thin film and the crystal state are also differentiated, crystal defects such as dislocation are generated in a silicon single crystal substrate, electrical properties of a semiconductor device are differentiated in a wafer including a silicon single crystal, etc.
It is an object of the present invention to provide a semiconductor device improved in reliability overcoming the defects as mentioned above and processes for producing such a semiconductor device in a high yield.
It is another object of the present invention to provide processes for producing a polycrystalline silicon thin film on an optional substrate and a chemical vapor deposition apparatus for forming such a silicon thin film.
The present invention provides a semiconductor device comprising a semiconductor substrate, an underlying film formed thereon and a silicon thin film doped with an impurity selected from group III and V elements and formed on the underlying film, crystal grains of said silicon thin film having mainly a columnar structure grown from an interface of the underlying film to a surface of the silicon thin film, and a crystal orientation on film surfaces of individual crystal grains being in an almost uniform direction.
The present invention also provides a process for producing a semiconductor device, which comprises forming an underlying film on a semiconductor substrate, and forming a silicon thin film on the underlying film by depositing a silicon film having no impurity from a SiH4 gas or a Si2H6 gas to a thickness of 1 nm or more, followed by deposition of the silicon film doped with an impurity selected from group III and V elements. When an amorphous silicon thin film is deposited, heat treatment is conducted to finally provide a polycrystalline silicon thin film.
The present invention further provides a process for producing a semiconductor device, which comprises forming an underlying film on a semiconductor substrate, forming an impurity layer from an impurity gas selected from group III and V elements, said impurity layer having a higher concentration of impurity than an average impurity concentration in a silicon thin film to be formed on an interface of underlying film, and depositing a silicon film from a SiH4 gas or a Si2H6 gas doped with the impurity. When an amorphous silicon thin film is deposited, heat treatment is conducted to finally provide a polycrystalline silicon thin film.
The present invention still further provides a process for producing a silicon thin film, which comprises introducing into a reaction chamber a raw material gas selected from SiH4 gas and Si2H6 gas to deposit a silicon film having no impurity to a thickness of 1 nm or more, followed by introduction of an impurity gas selected from group III and V elements together with the raw material gas to deposit a silicon film doped with the impurity. When an amorphous silicon thin film is deposited, heat treatment is conducted to finally provide a polycrystalline silicon thin film.
The present invention also provides a process for producing a silicon thin film, which comprises introducing into a reaction chamber an impurity gas selected from group III and V elements to form an impurity layer having a higher concentration than an average impurity concentration in a silicon thin film to be formed on an interface of underlying film, and introducing a raw material gas selected from SiH4 gas and Si2H6 gas together with the impurity gas to deposit a silicon thin film doped with the impurity. When an amorphous silicon thin film is deposited, heat treatment is conducted to finally provide a polycrystalline silicon thin film.
The present invention further provide a chemical vapor deposition apparatus for forming a silicon thin film comprising
a reaction chamber,
a gas introducing unit for introducing a raw material gas and an impurity gas into the reaction chamber,
a unit for controlling film deposition in the reaction chamber, and
a gas exhaust unit for exhausting gases from the reaction chamber,
said unit for controlling film deposition being made either
(i) so as to introduce the impurity gas selected from group III and V elements together with the raw material gas after the introduction of only the raw material gas selected from SiH4 gas and Si2H6 gas for a predetermined time, or
(ii) so as to introduce only the impurity gas selected from group III and IV elements for a predetermined time before the introduction of the raw material gas selected from SiH4 gas and Si2H6 gas together with the impurity gas.